Microscopic struture of amorphous covalent alloys probed byab initiomolecular dynamics: SiC

Radiation-Induced Topological Disorder in Irradiated Network Structures

“…keeping [SiC 4 ] coordination tetrahedra intact) is impossible to achieve. It is also consistent with analyses of an ab initio MD-simulated amorphous SiC [114] and of Tersoff's a-SiC assembly modeled with a smooth cutoff [113], which give χ = 0.6 and 0.5 respectively. Recently completed MD simulations, carried out by Professor Yip's group [125] in parallel with the topological MD results reported here, have corroborated the role of a critical local homonuclear bond density in effecting amorphization.…”

“…These annealed structures are then cooled down to room temperature and analyzed. [114] also suggest that there are less than 70% 4-fold Si sites in a-SiC. Despite the great variation in numbers for the different approaches, it is clear that a significant portion of Si sites are not in tetrahedral configurations.…”

“…The divergences at large χ result from the inability of the structure to undergo meaningful structural adjustment at temperatures as low as 300 K. The initial fall in density and rise in energy result directly from the introduction of isolated antisite defects. Calculation from the initial energy-rise slope yields a formation energy of 7.5 eV for an antisite pair (C Si + Si C ), compared to 7.2 eV from an earlier calculation using the 1989 Tersoff potential [119] and 8.4-8.5 eV from an LDA ab initio calculations [111,114]. Deviations from the initial slopes of either curve occur when antisite coordination shells start to overlap at associated antisites that require lower formation energy.…”

“…Chemical disorder χ, defined as the ratio of C-C bonds to C-Si bonds, N C-C /N Si-C [113], is not a complete homonuclear bond analysis and is specified only for C atoms. The argument for using χ instead of R hnb derives mostly from the practical uncertainty of enumerating Si-Si bonds in amorphous structures for which Si atoms have no clearly defined first coordination shell [112][113][114]. In practice, χ is quite a good approximation to R hnb , and the deviation is proportional to the CN difference between Si and C; when all atoms have the same CN, χ = R hnb .…”

“…Compared to earlier AIMD study of a-SiC [114], MD simulation with any version of Tersoff empirical potentials yields significant differences in C/Si local configurations. For example, our MD simulations of a-SiC with the various Tersoff potentials [117] generated 26% (1989), 80% (1990), and 33% (1994) tetrahedral Si sites, whereas AIMD gives 65-70% tetrahedral Si sites [114]. In addition, Si-Si pair radial distribution functions and homonuclear bond ratios differ in empirical-potential (classic) MD and AIMD simulations.…”